HUBBLE’S FAMOUS IMAGES OF GALAXIES typically show them to be elegant spirals or soft-edged elliptical shapes.
But these neat forms are only representative of large galaxies. Smaller galaxies like the dwarf irregular galaxy Holmberg II come in many shapes and types that are harder to classify.
Holmberg II’s indistinct shape is punctuated by huge glowing bubbles of gas, captured in this image from the Hubble Space Telescope.
The intricate glowing shells of gas were formed by the energetic life cycles of many generations of stars. High-mass stars form in dense regions of gas, and later in life expel strong stellar winds that blow away the surrounding material.
At the very end of their lives, they explode in as a supernova. Shock waves rip through these less dense regions blowing out and heating the gas, forming the delicate shells we see today.
Holmberg II is a patchwork of dense star-forming regionsand extensive barren areas with less material, which can stretch across thousands of light-years.
As a dwarf galaxy, it has neither the spiral arms typical of galaxies like the Milky Way nor the dense nucleus of an elliptical galaxy.
This makes Holmberg II, gravitationally speaking, a gentle haven where fragile structures such as these bubbles can hold their shape.
A hidden black hole?
While the galaxy is unremarkable in size, Holmberg II does have some intriguing features. As well as its unusual appearance—which earned it a place in Halton Arp’s Atlas of Peculiar Galaxies, a treasure trove of weird and wonderful objects—the galaxy hosts an ultraluminous X-ray source in the middle of the three gas bubbles in the top right of the image.
There are competing ideas as to what causes this powerful radiation—one intriguing possibility is that an intermediate-mass black hole is pulling in material from its surroundings, with the material giving off energy as it nears the black hole.
The colourful image is a composite of visible and near-infrared exposures taken using the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Hubble is a project of international cooperation between the European Space Agency and NASA.
Download the Hubble wallpapers:
Adapted from information issued by HEIC / NASA / ESA.
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